Conclusion: No refund. Cloudflare support ghosted them for 8+ months. The overcharges eventually balance out, but the prepaid billing model is fundamentally broken.
tldr: Cloudflare Images prepaid storage billing charged $400+/mo instead of the expected $110 — each capacity upgrade charges the full new amount upfront, but credits for the old capacity only appear on the next billing cycle. Cloudflare support was contacted in November 2023 and ghosted them for over 8 months.